Population exposure to heat-related extremes: Demographic change vs climate change

Thursday, 18 December 2014
Bryan Jones, CUNY Institute for Demographic Research, New York, NY, United States, Brian C O'Neill, National Center for Atmospheric Research, Boulder, CO, United States, Claudia Tebaldi, Climate Central, Princeton, NJ, United States and Keith W Oleson, NCAR, Boulder, CO, United States
Extreme heat events are projected to increase in frequency and intensity in the coming decades [1]. The physical effects of extreme heat on human populations are well-documented, and anticipating changes in future exposure to extreme heat is a key component of adequate planning/mitigation [2, 3]. Exposure to extreme heat depends not only on changing climate, but also on changes in the size and spatial distribution of the human population. Here we focus on systematically quantifying exposure to extreme heat as a function of both climate and population change.

We compare exposure outcomes across multiple global climate and spatial population scenarios, and characterize the relative contributions of each to population exposure to extreme heat. We consider a 2 x 2 matrix of climate and population output, using projections of heat extremes corresponding to RCP 4.5 and RCP 8.5 from the NCAR community land model, and spatial population projections for SSP 3 and SSP 5 from the NCAR spatial population downscaling model. Our primary comparison is across RCPs - exposure outcomes from RCP 4.5 versus RCP 8.5 - paying particular attention to how variation depends on the choice of SSP in terms of aggregate global and regional exposure, as well as the spatial distribution of exposure. We assess how aggregate exposure changes based on the choice of SSP, and which driver is more important, population or climate change (i.e. does that outcome vary more as a result of RCP or SSP). We further decompose the population component to analyze the contributions of total population change, migration, and changes in local spatial structure.

Preliminary results from a similar study of the US suggests a four-to-six fold increase in total exposure by the latter half of the 21st century. Changes in population are as important as changes in climate in driving this outcome, and there is regional variation in the relative importance of each. Aggregate population growth, as well as redistribution of the population across larger US regions, strongly affects outcomes while smaller-scale spatial patterns of population change have smaller effects.

[1] Collins, M. et al. (2013) Contribution of WG I to the 5th AR of the IPCC

[2] Romero-Lankao, P. et al (2014) Contribution of WG II to the 5th AR of the IPCC

[3] Walsh, J. et al. (2014) The 3rd National Climate Assessment